Aerogels are low-density, high surface area solid materials, typically ceramic oxides, which have been expanded using an explosive release of pressure, typically in a supercritical fluid (SCF) or by flash evaporation of a solvent from a sol-gel precursor solution. One of the more common aerogels is composed of silicon dioxide (or “silica”), which is presently available from a variety of commercial vendors. Aerogels commonly display remarkably high surface areas, achieved at minimal cost due to the simplicity of the method used for their synthesis. For example, silica aerogels exhibiting surface areas of approximately 1,250 m2/g, are commercially available. No time-consuming and expensive templating process is necessary for the manufacture of aerogels, as both the flash evaporation and SCF routes for their synthesis are readily amenable to large-scale production.
The high surface area exhibited by aerogels suggests their use in a variety of scientific and industrial applications. However, as a result of these limitations on the interfacial chemistry of the aerogel backbone, the utility of aerogels has been severely reduced, and aerogels have not found widespread use in applications where materials having a high surface area would present advantages.
For example, aerogels are typically very fragile structures, rendering them unsuitable in applications where a high surface area material is only useful if it is able to withstand an applied force, even as slight a force as the capillary force of a liquid. Also, in many applications, a material having both a high surface area and exhibiting specific chemical properties is desired. In many instances, the aerogels will fail to provide the specific chemical properties necessary for a given application. To overcome both of these drawbacks, many having skill in the art have attempted to provide coatings for aerogels. The ability to chemically modify the internal surfaces of an aerogel would provide direct access to inexpensive, high-surface area materials useful in a variety of uses, including, without limitation, as sorbents, catalysts and sensor materials. In this manner, it has been proposed that the aerogels could be made to exhibit enhanced strength and/or that aerogels could be made to exhibit chemical properties desired for a particular application by coating the internal and external surfaces of the aerogels with materials bonded on one end to the aerogel, and having a molecule with desired chemical or “functional” properties at the other end.
Unfortunately, attempts to provide coatings on aerogels have so far met with little success. Traditional synthetic coating methods utilizing liquid carriers and the like have been unable to effectively coat the broad expansive surface area of aerogels for a variety of reasons. The random structure of the aerogel has a significant number of constrictions and/or blockages that hinder mass transport into the complex pore structure. Further, due to the high temperature nature of the synthetic protocol typically used to make aerogels, there is very little adsorbed water within the aerogel. Thus, in silica aerogels for example, the surface silanol population is quite low. This severely limits the amount of silane that can be bound by this surface. Also, as noted above, the ceramic oxide wall structure of the aerogels is extremely thin. As a condensed liquid phase enters the pore structure, the capillary forces brought about by liquid column in the tiny pores can overcome the fragile strength of the aerogel wall, thereby crushing the internal structure of the aerogels simply by filling it with liquid.
Thus, there exists a need for aerogels coated with strength enhancing monolayers and functionalized monolayers, and methods for coating aerogels with strength enhancing monolayers and functionalized monolayers